Xylenes are aromatic hydrocarbons that include a benzene ring and two methyl substituents. Based on the structural position of the methyl substituents, three isomers of xylene can be formed: paraxylene, metaxylene, and orthoxylene. Paraxylene is a feedstock for terephthalic acid, which is used in the manufacture of synthetic fibers and resins. Metaxylene is used in the manufacture of certain plasticizers, azo dyes, and wood preservatives. Orthoxylene is a feedstock for phthalic anhydride, which is used in the manufacture of certain plasticizers, dyes, and pharmaceutical products.
For production of a desired xylene isomer, a mixed stream of the three xylene isomers is typically produced before the desired xylene isomer is separated. In other words, the desired xylene is not selectively produced but is selectively separated. A desired xylene isomer can be separated from mixed xylene streams by using an adsorbent selective to the desired isomer. After the desired isomer is adsorbed from the mixed xylene stream, the remaining isomers are discharged in a mixed raffinate stream. Typically, a desorbent desorbs the desired xylene isomer from the adsorbent, and the desorbent and selected xylene isomer are collected and separated by fractionation.
The market for petrochemical products dictates the ratio of a selected xylene isomer to benzene to be achieved in an aromatics complex. To achieve a desired ratio of a selected xylene isomer to benzene, particular units in the complex must be made larger and run more severely to convert the feed to products yielding the desired ratio of a selected xylene isomer to benzene resulting in large capital and operational expense.
Accordingly, it is desirable to provide processes for more easily achieving a ratio of a selected xylene isomer to benzene, particularly in an aromatics complex.